The present invention relates generally to a device for generating a subatmospheric pressure, and more particularly, to a device for generating a subatmospheric pressure at a coating layer on a web-shaped carrier material which is guided over a rotating coating roll.
U.S. Pat. No. 4,445,458 discloses an extrusion coating device for applying a coating solution onto a web material moving over a roll. The device employs a drawdown die which has an oblique drawdown surface, the angle between the drawdown surface and the axis of the extrusion slit being an obtuse angle. The extrusion coating device is equipped with a subatmospheric pressure box which, at a relatively high rate of conveyance of the web material, is intended to reduce the entrained air so as not to destroy the contact of the meniscus of the coating solution with the web. The subatmospheric pressure is generated outside the subatmospheric pressure box by means of an evacuating pump which is not described in detail. The structure of the box and the manner in which it is sealed are also not described in detail.
U.S. Pat. No. 2,681,294 describes processes for coating web materials using a slip film coating. The coating devices used each possess a subatmospheric pressure box which is connected to a drainage line in order to drain off excess coating solution. In one embodiment the coating device with the subatmospheric pressure box and a dryer are disposed in a superatmospheric chamber. The subatraospheric pressure box of the coating device has a connection to the ambient air, the cross-section of this connection being controllable so that the differential pressure at the meniscus of the coating solution is adjustable.
The device described in DE-A 3 309 343 for applying at least one casting layer on a moving, web-shaped substrate possesses a rotating casting roll which guides the substrate. A casting device is arranged close to the casting roll, separated only by a gap. A suction device with a housing has a subatmospheric chamber extending from the gap as far as a dividing wall, and a suction chamber extending from the dividing wall as far as an end wall. The dividing wall and/or the end wall are adjustable in the circumferential direction. The subatmospheric pressure chamber and the suction chamber can be connected together via a bypass. By displacing the dividing wall and/or the end wall, the volumes of the subatmospheric pressure chamber and the suction chamber are changed. These volumes, together with the bypass and the gaps to the coating web, form a pneumatic oscillating system. By changing the volumes, resonant frequencies can be changed and, by varying the bypass, the damping can also be changed. In the subatmospheric chamber there is disposed a rotating air roll which represents a source of disturbance for the uniformity of the coating.
Document EP-B 168 986 describes a slip film coating with subatmospheric pressure together with tangential application of the coating solution. The slip film flows downwardly on a perpendicular wall and passes via a gap onto the web to be coated. The web is deflected at this point over a roll so that the coating film lays itself tangentially against the web. The gap is connected to a subatmospheric chamber so that the coating can be influenced. A defined subatmospheric pressure or a defined superatmospheric pressure can be applied via a switch unit. The subatmospheric pressure aids the coating, while the superatmospheric pressure leads to an immediate interruption of the coating. The pressure chambers lies in the direct vicinity of the coating gap between the leading end of the slip web surface and a coating roll over which the web material is guided. The coating gap is at a position with respect to the coating roll such that both the supported web material and the material to be coated move downwardly at this position. The pressure chamber is arranged above the coating gap, and the angle between the lowermost section of the slip web and the tangent onto the coating roll in the region of the coating gap is between 170.degree. and 180.degree..
In known subatmospheric pressure systems for coating moving webs, the subatmospheric pressure space is separated from standard pressure in the region of the coating roll and of the web material by one or more narrow gaps or by adjacent seals. The adjacent seals can lead to wear or surface defects in the coating roll or the web material. Depending on the gap width, gap seals will allow relatively large amounts of air to enter the sub-atmospheric pressure box. This air must then be removed via the subatmospheric pressure generation system. The amount of air entering the box varies according to the width and thickness of the web material, so that the subatmospheric pressure or the gap seal must be reset in each case. Additionally, the presence of air can cause turbulence and oscillations which in turn can impair the coating meniscus, that is to say the application bead of the coating solution.
In known systems for coating under subatmospheric pressure, there is also the danger that the coating solution can be drawn off when there are instabilities in the subatmospheric pressure system. Drying out of the solution results in coating streaks which can also cause oscillation, again leading to pulsations in the sub-atmospheric box or at the coating meniscus.
Moreover, with a sheet die, high web rates with simultaneously thin coating layers can only be achieved if subatmospheric pressure is applied. The subatmospheric pressure acts from the side of the still uncoated web on the coating film between the die outlet and the web. The subatmospheric pressure is generally generated separately from the coating region via one or more injection nozzles. The air to be drawn out of the subatmospheric pressure box is led via hoses to the injection nozzles. At relatively high subatmospheric pressure, the coating film breaks up between the sheet die and the web and can only be made continuous again at low web rates. The causes of the breaking up are oscillations in the subatmospheric pressure box and in the suction lines, and cross-flows caused by leaking air.